1. Field of the Invention
The invention relates to a circuit-breaker comprising two contacts disposed in a breaking space containing a dielectric gas and between which an electrical arc is established during an operation of opening the circuit-breaker, which includes a thermal blast chamber communicating with the breaking space.
2. Description of the Prior Art
The invention applies to a high-voltage circuit-breaker intended to break high currents and at the same time to minimize the duration of an electrical arc appearing between its contacts during the opening operation. In this context the expression xe2x80x9chigh currentxe2x80x9d refers to a current that has a high amplitude or continues to flow for a long time period. In this regard, the invention applies in particular to breaking low-frequency alternating currents, for example the currents flowing in railroad power supply networks which are supplied with power at a frequency of 16.66 Hz or 25 Hz, as in Germany and Switzerland, for example. With such low frequencies, the duration of the current wave is two to three times longer than for a frequency of 50 Hz, and the heat produced in the event of electrical arcing can therefore be two to three times greater than for a 50 Hz current.
To improve their breaking capacity, conventional circuit-breakers generally comprise a pneumatic self-blast device which blows dielectric gas in the direction of the arc at the time of opening to encourage breaking space of the arc. A self-blast device of this kind conventionally comprises a compression chamber including a piston coupled to a mobile contact of the circuit-breaker to move with it and adapted to blow a constant volume of cool gas in the direction of the breaking space during each opening. The piston is moved by the circuit-breaker actuation energy causing the movement of the mobile contact during opening. The greater the electrical arc current, the greater the pneumatic blast required. To break high currents, the dimensions of the piston compression chamber must be increased accordingly, necessitating consequential uprating of the circuit-breaker actuator system so that it is capable of supplying sufficient energy for the blast. Using an uprated actuator system generates a high overcost, which makes the price of this kind of circuit-breaker relatively uncompetitive.
One way to increase the blast to break high-current arcs is for a heating blower chamber to adjoin the pneumatic compression blower chamber. In the heating blower chamber, which is situated between the pneumatic blower chamber and the breaking space, for example, the dielectric gas is heated by the electrical arc and its pressure increases. The thermal blast chamber is shaped to encourage the flow of the gas that it contains toward the breaking space in the event of an increase in the pressure of the gas, with the result that the higher the arc current, the greater the blast. Nevertheless, in the case of a high-current electrical arc, the temperature in the thermal blast chamber can increase excessively, which reduces the dielectric strength of the gas that is blown into the breaking space and makes it impossible to break the current.
U.S. Pat. No. 4,517,425 discloses a circuit-breaker comprising a thermal blast chamber which communicates with a breaking space via the throat of a nozzle and with an expansion space through passages closed by valves. In one particular embodiment of the circuit-breaker, at least one of the valves is an evacuation valve adapted to open when the pressure in the thermal blast chamber is greater than a particular threshold to evacuate pressurized gas from the chamber to the expansion space.
This construction increases the breaking capacity because the valve opens to depressurize the chamber if the pressure in the thermal blast chamber becomes too high. This depressurization decreases the temperature, which guarantees that the gas blown into the breaking space has a satisfactory dielectric strength.
In the above prior art circuit-breaker, the passage shut off by the evacuation valve is in the base of the fixed contact and opens into one end of the breaking chamber. It is therefore necessary to provide a dedicated expansion volume adjoining the breaking chamber. The evacuation of the gases then does not contribute to the blowing of the hot gases and/or the arc into the divergent section of the nozzle. Furthermore, it is evident that this type of arrangement is not suited to an architecture in which a pneumatic blast chamber adjoins the thermal blast chamber.
The patent document EP 0296363 discloses a circuit-breaker with pneumatic and thermal blast chambers. The wall separating the two chambers is spring-loaded and incorporates an opening that is arranged with another opening in the cylindrical casing that delimits the perimeter of the pneumatic blast chamber so that the two openings constitute a valve for evacuating the pressurized gases coming from the thermal blast chamber.
The separator wall is adapted to compress the spring if the pressure in the thermal blast chamber is higher than that in the pneumatic blast chamber, so that the openings can be lined up to allow the pressurized gas to escape toward an expansion space around the cylindrical casing common to the pneumatic and thermal blast chambers. Note, however, that a device of this type cannot be applied to a circuit-breaker architecture in which permanent current contacts are disposed around blast chambers, i.e. in the expansion space for the hot pressurized gases. This is because, at the moment the arc contacts separate, the gas between the permanent current contacts would no longer have sufficient dielectric properties to prevent, arcing between the permanent current contacts, which are not designed to withstand arcing.
An object of the invention is to remedy the above drawbacks by proposing a circuit-breaker capable of breaking high currents thanks to a system for evacuating hot pressurized gases, without this necessitating excessive modifications to the architecture of a conventional circuit-breaker. In particular, the aim is to provide a device such that only a very small number of components need to be modified in a standard circuit-breaker not designed to withstand such high currents with equally long wave durations/periods (or equally low frequencies).
To this end, the invention proposes a circuit-breaker including two contacts disposed in a breaking space which is delimited by a blast nozzle and contains a dielectric gas, the circuit-breaker including a thermal blast chamber which communicates with the breaking space via a throat of the nozzle and with an expansion space via an evacuation passage adapted to be shut off by a valve, the valve being adapted to open when the, pressure in the thermal blast chamber is greater than a particular threshold to evacuate the pressurized gas from the chamber, which circuit-breaker is characterized in that the evacuation passage is formed in the nozzle and defines a circular volume within the thickness of the nozzle, following the general shape thereof, and opening into the expansion space downstream of the breaking space relative to the throat.
With this form of construction, the blowing of gas through the evacuation passage contributes to blowing the hot gases contained in the divergent section of the nozzle and therefore improves the regeneration of the dielectric strength in the breaking space after the arc is extinguished.
The nozzle can preferably also incorporate the valve and, to offset development and fabrication costs, can therefore be retrofitted to an existing circuit-breaker.
In another particular embodiment of the invention, the nozzle comprises two coaxial parts, its external part surrounding its internal part to leave a circular free space forming an evacuation passage for evacuating gas out of the thermal blast chamber, the valve being designed to shut off said evacuation passage. This makes the nozzle less costly to fabricate. The valve and an associated evacuation passage define a circular shape to reduce head losses. Thus the evacuation passage can accommodate a high flowrate to reduce the pressure rise in the thermal blast chamber as quickly as possible. The valve can advantageously have an annular shape and bear on one or more calibrated springs, so that it opens against the action of the spring(s). The valve opening threshold can therefore be altered merely by changing the calibrated spring(s).
In another particular embodiment of the invention, the circuit-breaker includes a piston compression chamber that communicates with the heating compression chamber. The gas blown into the breaking space is then a mixture of cool gas from the piston compression chamber and hotter gas from the thermal blast chamber, which reduces the temperature of the gas to maintain a high breaking capacity of the circuit-breaker.
The invention is described next in more detail and with reference to the accompanying drawings, which show one embodiment of the invention by way of non-limiting example.